In the manufacture of long concrete panels on elongated casting beds, one manufacturing step involves removing of pieces of the cast concrete plank from the casting bed after the plank is cast and partially cured. A method of accomplishing this function, which was used in the prior art, was the insertion of lifting loops into the top surface of the panel. The lifting loops, formed of pieces of reinforcing bar or loops of prestressing cable, projected above the top surface of the panel. These loops could be readily engaged by hooks on the ends of cables suspended from a crane to provide for the lifting of the planks.
In casting processes where hollow core planks are manufactured by a slipform technique to form voids by temporarily filling them with aggregate, it is necessary to dump this core forming material from the planks after they are removed from the casting bed. Although this operation has been done by using lifting loops, the loops are susceptible to pulling out of the concrete during lifting. Further, the projecting portion of the lifting loops must be removed for appearance sale after installation of the plank at a construction site. Covering the remnants of lifting loops after the panel has been erected to form building walls or floors is labor intensive and tends to lead to staining as the panel weathers.
In U.S. Pat. No. 4,398,761, assigned to the same assignee as the present invention, a device for lifting and tilting concrete plank or panels is described. The device has a pair of clamping heads suspended from opposite ends of an extendable beam. The device is a significant improvement over lifting loops for lifting and tilting concrete panels. However, the very high weight of the concrete panels results in a very high torque at the interface of the concrete panel and the clamping head. Even a very slight slippage at the interface tends to cause the concrete to crack and spall. Such spalls are unsightly, and if repairable, repairs are expensive. If the spalling is severe enough, salvage of the plank may not be possible.